The diagram below summarises some of the key ways trees in the boreal forest have adapted to the abiotic factors of this area
Case study – The fate of Siberian forests Background information
Total area = 8.8 M sq km; (57% world Boreal Forest).
The Siberian approach to forests! In Boguchany, Siberia, 20,000 prisoners are set to work logging for punishment – the resultant timber is not used, its purpose was merely to occupy prisoners time. This represents a criminal waste of forest!
Such deforestation devastates local ecosystems and reduces wildlife food sources for indigenous people.
To make better use of the logs, the Boguchany dam (a local HEP project) is being built for processing logs. This at least reduces waste, but puts further stress on the forest… more forest destruction will occur by flooding for the reservior.
Meanwhile in the neighbouring region of Bratsk, Siberia, 100,000 Ha forest has been destroyed by air pollion from aluminium smelters, power stations and chemical factories. This also affects humans ….local mortality rates increased 25% in last 10 yrs.
Temperate forest case studies
The Forestry Commission in the UK are pursuing MPF (multipurpose forestry). Timber, employment, landscapes, watersheds, biodiversity, habitat considered together (but timber production takes priority!).
UK has 10% tree cover but only 2.5% native/seminative (since conifers in UK have 2-3 x broadleaved growth rate so are planted commercially.
A mere 30,000 Ha of broadleaved forest is coppiced in UK but the trend is rising (see below)
Plans and projects:
Trees for Life group based in Scotland plan a 1,500sq km “Central Highland Forest” with wolves etc to recreate native forest on a grand scale.
Sussex wildlife trust – plans for a 5,000 Ha Weald Forest – large scale unmanaged controlled by natural grazers inc wild boar. “Minumum intervention regime” from which some timber cropped.
National Forest – (FC & Countryside Commision) – planned a 500sqkm in Midlands for MPF with an aim to have 30% forest cover within the designated area.
A growing market is charcoal. Britain uses an estimated 60 000 tonnes a year, mostly for barbeques. Much of it imported from endangered tropical mangrove swamps. In recent months, stores ranging from Harrods to B&Q, the home improvement chain, have agreed to supply charcoal from British wood. Coppiced wood is also still used for a variety of jobs, from fencing and thatching to making barrel hoops, tent pegs and walking sticks.
Every tonne of dry coppiced wood could generate as much electricity as 650 kilograms of coal, and save the release of up to 500 kilograms of carbon into the atmosphere
UK Government recently gave the go-ahead for three wood-burning plants to produce 19 megawatts from coppice wood and forest waste. That implies an initial 5000 hectares of coppices, says Damian Culshaw of ETSU. The largest project is the Arbre power station. This 75 million project, headed by Yorkshire Water, is backed by the EU and companies from France, Germany and Sweden
The 8- megawatt plant will turn wood chips into a mixture of carbon monoxide and dioxide, hydrogen and methane, while a new catalytic process will break big hydrocarbons into combustible compounds. The gases will then be burnt in a gas turbine, and the heat produced used to drive a steam turbine. This should produce about twice as much electricity from each tonne of wood as simply burning it and driving a steam generator,
This year some 100 000 square kilometres of pristine rainforest – an area roughly the size of Iceland – will be burnt to the ground to produce farmland. A further 50 000 square kilometres will be invaded by logging companies intent on extracting timber of high commercial value. Nobody doubts that the burning will cause irreversible destruction, of both trees and wildlife. But what of the logging? The conventional view of timber extraction is that it is the kiss of death for a rainforest. Loggers move in with their roads and heavy tractors, remove the most valuable timber and then abandon the forest, depleted, to slash-and-burn farmers. But does it have to be like this? The International Tropical Timber Organisation, a trade association set up by the 47 nations of the world that either produce or import tropical timber, thinks not. Driven by public concern and necessity – many of the world’s rainforests will be exhausted within two decades if things do not change – ITTO has become an evangelist for sustainable logging. It has set itself the target of making responsible, ‘scientific’ logging the norm throughout the tropical world by the year 2000.
Many environmentalists remain profoundly sceptical. They point to the ITTO’s abysmal track record on policing the timber industry and the fact that less than 0.1 per cent of the world’s rainforest is at present under any form of sustainable management. Everyone agrees that without enormous political will this situation is unlikely to change. But even if the will is there, how can we be sure that any future logging practices dubbed ‘sustainable’ by the timber industry really do allow rainforest the chance to regenerate? Underlying this question are some thorny scientific problems. ;UL?
* Just how resilient are rainforests to small-scale disturbances?
* How fast can they repair gaps in their canopies? And
* What, if any, is the ecological value of the ‘secondary’ vegetation which often emerges after a rainforest has been logged?
Five years ago we set about finding some answers in Borneo, home to one of the world’s richest rainforests. We joined a team of scientists from Britain and Southeast Asia at the Royal Society’s field station in Danum Valley, Sabah, one of the most heavily logged regions of Malaysia. Our mission was to study the impact on a small area of rainforest – its vegetation, microclimate, wildlife, insects and so on – of creating holes in its canopy. Recently we met other botanists and ecologists at the Royal Society in London to discuss in detail what we have found so far.
Dynamism in forest ecosystems
Tropical rainforest is commonly perceived as one of the most ancient and stable ecosystems on Earth. Yet even virgin, or primary, rainforest suffers much natural disturbance by, for example, hurricanes, landslides and even droughts. During the past decade, for example, tracts of primary rainforest in Borneo and Brazil were destroyed by fires brought on by drought. The great fire of Borneo in 1983 destroyed some 40 000 square kilometres of forest, a vast area given that the present rate of logging there is about 7000 square kilometres per year. Much more common, though, is disturbance of a less drastic kind – the tiny gaps that appear in a forest canopy whenever a tree dies and falls over. Anyone who has camped overnight in a rainforest will have been startled from time to time by a nearby tree crashing to the ground. Far from destroying the forest, such natural disturbances provide opportunities for regeneration. An entire rainforest can turn over in not much more than 120 years. If rainforests can recover from natural disturbances, why not from logging? Presumably the odds on recovery will be shortest when the gaps are small. But exactly how small is small, and to what extent does the size of a gap influence the speed and quality of forest regeneration? These questions lie at the core of a branch of biology known as forest dynamics, and they underlie much of our work in Sabah.
Forest dynamics is by no means a new subject. For several decades, biologists have studied forest regeneration in the natural gaps formed by the everyday collapse of ageing trees. One factor more than any other seems to determine the pattern of regrowth: the amount of sunlight the gap allows in through the forest’s evergreen canopy. When it is completely intact, the canopy – which can be up to 30 metres deep – absorbs most of the sunlight, leaving the forest floor cool, moist and dark. So intense is the shade, in fact, that the banks of seedlings which cover the rainforest’s floor find it hard to grow. Most of these seedlings are of so-called climax species, trees such as the dipterocarps which predominate in primary rainforest in Southeast Asia and provide valuable hardwood timber. Their seeds are produced almost continuously and germinate immediately. But in the deep shade of the forest floor, most of the resulting seedlings remain suppressed for years, scarcely growing at all. The tiniest hole caused by a branch falling may be enough to trigger the growth of a climax seedling from the forest floor. Yet give such a seedling too much sunlight and it will soon wither.
The pioneer problem
The risk of overdosing on light is not the only problem climax seedlings face in their struggle to reach maturity. They must also contend with stiff competition from another class of tropical plant: fast-growing trees with soft, low-density wood, known as pioneers. One such adversary is balsa, a pioneer species from Central America which can grow at up to seven metres a year. Unlike their climax counterparts, the seeds of pioneers can remain dormant in soil for years and usually germinate only after being activated by sunlight or heat. Also in contrast to climax seedlings, pioneers thrive in bright light. So in open sunny gaps, or following slash-and-burn clearance of a forest, pioneer vegetation soon emerges and dominates. A gap produced by the deaths of four or five adult trees allows at least 20 times as much sunlight to reach the forest floor – more than enough to stimulate the germination of pioneers. Grouping tropical trees into light-loving pioneers and shade-adapted climax species is the starting point for understanding forest dynamics. But this simple division conceals many complexities, not least the question of how rainforests evolved, and continue to sustain, such an exuberance of plant life.
A single hectare of Peruvian rainforest contains on average eight times as many species of tree as the whole of Britain. How can so many species coexist without a large proportion being driven to extinction? Why are there so many apparent winners in the competition for light, water and mineral nutrients? Answering this puzzle holds the key to developing strategies for exploiting rainforests sustainably.
Explaining rain forest diversity
Competing theories abound.
* One hypothesis, championed by the Costa Rican ecologist Dan Janzen in the mid-1980s, points to pressure from predators as the driving force behind rainforest diversity. Each time a herbivore species evolves protection against the chemical or mechanical defences of a plant, there will be evolutionary pressure on plants to evolve further defences. It is this biological arms race which, supposedly, ensures a constant turnover of new and more varied species of both plant and predator.
* A second school of thought ascribes the diversity of tropical ecosystems to the effects of natural disturbances. The most extreme theory of this kind is the so-called refugia hypothesis, which points to the climatic turmoil of the Pleistocene (between 1 and 2 million years ago) as the dominant source of disturbance. As glaciation advanced over the northern continents during that period, it argues, rainfall in the tropics diminished, with the result that what were previously whole forests temporarily fractured into islands – or refuges – of forest separated by arid areas. Life within these refuges then rapidly diversified; when they later merged the resulting habitats were rich in species and have remained so ever since. That, at least, is the speculation.
* In principle diversity may also be the product of less drastic disturbances to rainforest habitat, such as damage caused by landslides, earthquakes, rivers switching course, or even the collapse of aged or diseased trees. This is the basis of the so-called gap dynamics theory. The idea here is that the conditions prevailing in gaps of different sizes favour different species of tree. Hence each climax species is uniquely adapted to grow in a gap of one particular size. Moreover, because gaps caused by natural disturbances must vary in size, many different species of tree are encouraged to regenerate. Variety in gap size is the driving force behind biological diversity in the rainforest – or so the hypothesis holds.
Testing these theories has proved quite a challenge for tropical ecologists. What limited data there are seem to cast doubt on the importance of predator pressure, but the theory still has its committed disciples. Similarly, the evidence for the refugia hypothesis remains far from conclusive. Its critics point to the fact that many, if not most, rainforest species are distributed uniformly, rather than in clumps. Meanwhile, in the absence of any evidence at all, it has been the gap dynamics theory that has appealed most to ecologists.
Case study of tropical forest research program
What we did in Sabah was to cut a series of precise holes in the canopy of the Danum Valley rainforest. These varied in size from 10 to 1500 metres square, and during the following four years we carefully recorded the fate of each climax seedling growing on the exposed forest floor beneath the holes. Most of the work was done with Tim Whitmore, a rainforest ecologist from the University of Oxford. As expected, many of the climax seedlings in the largest gaps were scorched by strong sunlight and died within a few weeks. Others struggled on only to be massacred by shoot borers and other insects at a later stage. The survivors of this onslaught appeared unable to continue growing upwards, so the gaps soon filled with sprawling bushes. As we monitored the fates of climax seedlings, Don Kennedy and Mike Swaine of the University of Aberdeen kept tabs on the pioneers. Although these germinated in gaps of many different sizes, only in the larger gaps – formed by the removal of two or more canopy trees – was there enough sunlight for pioneer seedlings to survive more than a couple of years.
The most surprising finding came from the smaller gaps. At the outset we had expected gaps of different sizes to favour different climax species, in line with the gap dynamics theory. Yet our results provided scant evidence of this. What seemed to matter most was simply seedling height. Initially, each gap was populated by a variety of seedling species, all of very different ages and sizes – the survivors of many years of suppression by forest shade. But in due course it was the seedlings that were tallest to start with – regardless of species – which went on to dominate the gap. The smaller seedlings, even if they were able to grow faster than their taller colleagues in bright light, were unable to catch up before the ground became shaded by the expanding canopy of the tall seedlings. The tallest seedlings were usually of species such as Hopea nervosa, which can grow slowly in deep shade for many years. The seedlings of species less well adapted to shade, such as Parashorea malaanonan, were younger and shorter. The gap dynamics theory overlooks this type of variation in the seedling bank, which may explain why our results appear at odds with its predictions. At the same time, our findings raised a new puzzle: why do climax species which can survive deep shade not drive to extinction those that need light to grow?
We think the reason lies not in the different sizes of natural gaps but in a completely different variable: how frequently a rainforest canopy is opened up. Even the tiniest gap in the canopy substantially improves the survival prospects of seedlings that cannot grow in deep shade. So in areas of forest which are especially prone to disturbance, shade-adapted seedlings will lose their competitive advantage over seedlings that cannot grow in shade. The spells between disturbances are simply not long enough for them to gain a head start. The more frequently the canopy is opened up, the more the two kinds of seedlings would approach competitive equality. In some circumstances, the initial inequality may even reverse itself, because extreme tolerance to shade must involve at least some physiological compromises. Greenhouse studies in Costa Rica and Sabah show that in sunny conditions shade-adapted climax seedlings are generally outpaced by seedlings that struggle to survive in deep shade.
Implications for rain forest management
Taken together, our results sound a cautionary note for rainforest management.
At present the vast majority of logging operations around the world create large gaps in canopies. Figures from the Amazon and Southeast Asia indicate that as many as 70 per cent of trees in a forest may be damaged or destroyed to extract only 10 per cent of them. To make matters worse, bulldozers and caterpillar tractors often crush the forest’s banks of climax seedlings. Even when logging is done selectively, careless forestry practice generally destroys at least half the canopy. Only pioneer species are able to cope with the resulting brightness, heat and low humidity.
There are no easy solutions. Unless logging companies eliminate all unnecessary damage, they will have to reduce drastically the amount of timber they extract. If climax species are to be given any chance of regenerating, loggers must open up no more than 25 per cent of the canopy during each cut. Even then, the biological diversity of the forest will be at risk. To extract timber in a way that is both commercially and ecologically sustainable, logging companies must go a step further. They must try and mimic the natural disturbances that help to sustain biological diversity, and avoid patterns of activity that erode it. This means varying not only the sizes of the gaps they make but also how often they disturb different parts of the forest.
But the news is not all bad. Pioneer vegetation may be of lower biological diversity than primary rainforest, and its soft timber may be of low commercial value, but its speedy growth in a logged rainforest can play a vital part in restoring the damp, cool conditions necessary for climax trees to re-establish themselves. What is more, according to research by Ian Douglas and his colleagues from the University of Manchester, pioneer trees can also help to check the soil erosion that occurs when logging exposes forest land. Following a commercial cut in Sabah, the researchers found that the sediments in a stream draining the logged forest had increased 18-fold; but a year later, after pioneers had colonised the area, the sediments had fallen to less than four times the normal level.
Different impacts on different species
Research on the impact of logging on wildlife gives room for optimism, too. Contrary to expectations, Andy Johns and Frank Lambert, of the University of Aberdeen, have found that few species of vertebrate are lost entirely when a rainforest is logged, though the local populations of some vertebrates plummet. In their study, large herbivores such as elephants and deer positively thrived on pioneer vegetation. The creatures that fared least well were those with highly specific food needs. Woodpeckers and flycatchers, for example, became confined to whatever small pockets of untouched forest remained after logging. Primates were also hard hit. The local orang-utans had few young for several years after the forest had been logged. A similar story is emerging from the work of entomologists Jeremy Holloway of the Natural History Museum and Ashley Kirk-Spriggs of the National Museum of Wales. On the positive side, logging appears to leave relatively unscathed those insects such as dung and carrion beetles that can exploit a wide range of food resources. But the local populations of insects such as moths, which feed on highly specific rainforest plants, tend to crash.
The multiplier effects
All these results, however, paint what is probably an idealised picture. Logging invariably provides road access for hunters and migrant farmers who often devastate the local wildlife. Moreover, our knowledge of the intricate web of biological interactions that sustains a rainforest is still far from complete. A key unanswered question is the extent to which long-term fluctuations in the populations of pollinators, seed dispersers and herbivores affect the composition of a logged rainforest. What good does it do to preserve a forest’s capacity to regenerate its trees if logging has driven away the animals that disperse their seed?
What we do know is that natural disturbance is central to the life cycles of all rainforests, and that to get close to mimicking it, logging companies will have to reduce the size of the gaps they make in canopies. Yet, as our project also reveals, size isn’t everything: the success of the healing may also depend on the frequency with which the wounds are inflicted.
Nick Brown is a rainforest ecologist who lectures in the Department of Geography at the University of Manchester. Malcolm Press is a plant physiologist who lectures in the Department of Environmental Biology at the University of Manchester.
Tropical Forest – Key Issues
The tropical forest regions of the world are located approximately 10 degrees of latitude either side of the equator. They have excellent abiotic factors for plant growth (see diagram) so their potential Net Primary Productivity (NPP) is very high. Since trees are the dominant plant type in this biome the tropical forest has a very high biomass as well as a high NPP.
The rainforest biome is very diverse. There is a high biodiversity. A typical rainforest will have 4 distinctive layers ranging from the heliophytes (mainly trees) competing for light to the sciophytes below the canopy competing for space. This is shown in the diagram of tropical forest layering.
Many animals and insects have adapted to life in the trees since the maximum biological productivity is in the canopy, off the ground. The rainforest climate is very stable. Light, moisture, warmth and nutrients are all guaranteed to be in a bundant supply so productivity is enormous. This creates its own problems in terms of competition. The competition is both interspecific and intraspecific. Plants and animals adapt to the first by developing their own ecological niches. They adapt to the second type of competition by having generally low reproduction rates. This ends up producing very diverse forest where it is unusual to find clusters of similar trees. Instead, there are many different trees in the same area.
This adaptation is very good for reducing self competition but makes species very vulnerable if humans change the forest…… rates of recolonisation are very low.
There are many threats to the rainforest. Humans use the forest for many purposes. Traditional uses of the forest are sustainable with small population numbers. These include hunting/gathering; small scale slash and burn cultivation and rubber tapping.
Permanent cultivation is extremely damaging to the forest because the nutrients are all stored in the biomass therefore once the trees are cleared the nutrient level drops dramatically. Farming fails after a short time and the soil (once protected by vegetation) becomes severely eroded. Rivers become choked with eroded soil causing floods in wetter months. Conversely, the local climate dries out since there are no trees to transpire water back into the air. Deforestation of the tropical forest biome results in the twin curses of flood and drought.
Most developments taking place in the rain forest are very destructive to the forest ecosystem. The economic gains are usually very short term and often benefit relatively few people. One of the problems is that the forest is seen by governments as a “free” resource to be used for quick economic benefit. Both wealthy countries (EMDCs) and poor countries (ELDCs) abuse the tropical forests. The demands of western banks for “debt servicing” forces poor countries to sell their forest resources. The huge demands countries like Japan make for tropical wood products tempts poor countries to log their forest resources. For many ELDCs the forest is a short term political convenience. In Brazil the forest has been cleared and given to peasants to avoid the government having to tackle the politically unpopular subject of unjust land distribution. Forest indians have less political clout than wealthy – or greedy – landowners. In Malaysia a Minister for the Environment had major stakes in the timber companies. Some of the key pressures on rain forests are shown in the diagram above right. From your research it is important to find some named and quotable examples of each of these processes.
The management of rainforests is full of conflict. Much of the conflict depends on the value judgements that individuals take to the following questions:
* Who does the forest belong to? – Indians? Government? Landowners? Industrialists? Logging companies? Peasant subsustence farmers? Or does the ecosystem belong to itself?
* What is the value of the forest? – Global climate control? Regional flood and drought control? Biodiversity for ecosystem stability? Biodiversity for industrial/commercial use? Food source for locals? Timber source for export? Grazing for agribusiness? Plantations for industrial crops? Small scale rubber extraction? Source of HEP energy? Raw materials for mining? Wilderness for aesthetic and spiritual refreshment? Scientific laboratory for biological studies? Potential tourist resource for boosting economy?…….
* How is the forest best managed? – by Governments? by locals? by landowners? by timber companies? by environmentalists? The answer to this question depends largely on the answer to the question above. Key ways of managing forests are by
o Timber companies developing “sustainable forestry” – the Tropical Forestry Action Plan (TFAP)
o Environmental groups creating “Biosphere reserves” or negotiating “Debt for Nature” swaps – paying some of the country’s debt in exchange for reserve areas being set aside
o Multiple zoning reserves being created with a variety of uses to satisfy the conflicting values positions in the forest
o Promoting “sustainable harvesting” schemes (eg the Body Shop) to create economic growth from trading in tree produce (nuts, leaves etc) rather than trees
There are no easy answers to the problems of rainforest management. Too often the victims are portrayed as the criminals but the western nations are as responsible for the processes leading to deforestation as the developing countries are themselves. If global warming continues to alter world climates we may all end up paying dearly for our inactivity.